Rocks are all around us - you can see them in the countryside as cliffs or
boulders, in the city as parts of buildings and roads and at the seaside as
pebbles. Rocks are made up of minerals; some rocks, like limestone, are made up
of only a few types of mineral whereas others, like granite, are made up of lots
of different minerals which you can sometimes see as crystals.
There are three types of rock; sedimentary, igneous and metamorphic. Sedimentary
rocks are made of compacted particles of other rocks or mud which is usually
deposited in lakes and rivers as layers. As the layers build up the pressure
increases and the sediment is squeezed into rock. Limestone and coal are
examples of sedimentary rocks.
Igneous rocks are produced by volcanic processes; lava erupted from volcanoes
and magma (molten rock) held in a chamber beneath a volcano cools into igneous
rock. Igneous rocks like granite often have big crystals which grow slowly as
the magma cools whereas rocks that have formed from erupted lava cool very
quickly and have very tiny crystals, giving a glassy appearance.
Metamorphic rocks form when rocks deep in the ground are squeezed during
mountain formation, or when they get very hot. The heat, resulting from the squeezing
pressure, causes the rock to recrystallise. Marble is a metamorphic rock; it
forms when limestone, a sedimentary rock, is squeezed so hard that its crystals
Want to learn more about the formation of rocks? Click here to see a diagram
of the rock cycle. If you
want to try and identify your own rock
using the rock key click